With the type of voltage regulators in use today this should not generally be a problem.
If the printer board has some odd style it might be possible but unlikely.
FYI: computers and Pi’s provide +5v on the usb bus (this is part of the usb spec)
most devices plugged into such machines receive and are powered by that 5v
some devices also have their own power supply and may connect this to the same pin on the USB port.
The problem would come if both devices didn’t provide the same voltage…modern regulators are good but still not perfect so one might make 5.05v while the other 4.95v…In a straight short circuit this would allow power to run from the higher to the lower in a ‘backward’ fashion.
Buck converters which are commonly used to provide 5v from computers and printers and other power supplies switch on and off quickly: see https://en.wikipedia.org/wiki/Buck_converter
I like to use plumbing as an analogy for electrical systems…If you think of water pressure as Voltage and if you have a valve with high pressure on one side and you want to raise the pressure on the other you could open the valve a little and water would flow through from the hi side (supply) to the low side…you could just leave that valve open a little as as long as the water has some place to go on the low side you could set the pressure by balancing the water going in with the water going out…
But suppose as in our USB 5v case the low side actually had more pressure then the water would flow backward this would be bad imagine that is the sewer side of your water supply…yuck!
With the buck converter power supply however the valve is not just left open at some amount to raise the voltage on the low side to 5V rather it is opened and closed very fast and it generally has a one way valve and a sensor on the low side so that it won’t open unless that side is lower than voltage you want to set…so it opens and lets some water through then closes as soon as the desired pressure is reached. If the water is already at a higher pressure than it wants to set then it never opens. The only problem is if the pressure is so high it breaks the valve…but that takes many volts (lots of pressure) and there should never be that much difference between the two regulators, so things are safe.
Obviously this is a simplified description and there are various circuits that could be involved that might cause a failure but in general not a problem as evidenced by the fact that many people have many devices hooked up this way and it is seldom a problem (I would be curious to read the threads where this was a concern)…most common exceptions I can think of could be poorly rated parts (valves break way too easy) or really sub-standard designs (uses some other type of regulator) but these should be rare. Even in cheaper equipment these days uses standard parts and designs or copies of those and generally most use this type of converter.
end lecture mode…
